Improved quantification of grain boundary segregation by EDS in a dedicated STEM

Grain Boundary Plane Measurement Using Transmission Electron Microscopy Automated Crystallographic Orientation Mapping for Atom Probe Tomography Specimens

Grain boundaries are critical in determining the properties of materials, including mechanical stability, conductivity, and corrosion resistance. The specific properties of materials depend not only on the misorientation of the crystals, the three most commonly characterized parameters, but also on the angle of the grain boundary plane between the two crystals, the final two parameters in the five-parameter macroscopic description of the grain boundary. The method presented here allows for the direct measurement of all five parameters of the grain boundary in a transmission electron microscopy specimen of various morphologies. This is especially applicable to atom probe specimens, where only a single-tilt axis is generally available, allowing the crystallographic description to be matched to the detailed chemical data available in the atom probe tomography. This method provides a platform for efficient grain boundary analysis in unique samples, saving operator time and allowing for ease of acquisition and interpretation in comparison with traditional electron diffraction methods.

Evolution in X-ray Analysis from Micro to Atomic Scales in Aberration- Corrected Scanning Transmission Electron Microscopes

X-ray analysis is one of the most robust approaches to extract quantitative information from various materials, and is widely used in various fields ever since Raimond Castaing established procedures to analyze electron-induced X-ray signals for materials characterization 70 years ago. The recent development of aberration-correction technology in a (scanning) transmission electron microscopes (S/TEM) offers refined electron probes below the Å level, making atomic-resolution X-ray analysis possible. In addition, the latest silicon drift detectors (SDDs) allow complex detector arrangements and new configurational designs to maximize the collection efficiency of X-ray signals, which make it feasible to acquire X-ray signals from single atoms. In this review paper, recent progress and advantages related to S/TEM-based X-ray analysis will be discussed: (1) progress in quantification for materials characterization including the recent applications to light element analysis, (2) progress in analytical spatial resolution for atomic-resolution analysis and (3) progress in analytical sensitivity toward single atom detection and analysis in materials. Both atomic resolution analysis and single atom analysis are evaluated theoretically through multislice-based calculation for electron propagation in oriented crystalline specimen in combination with X-ray spectrum simulation.

Nanoscale light element identification using machine learning aided STEM-EDS

Light element identification is necessary in materials research to obtain detailed insight into various material properties. However, reported techniques, such as scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDS) have inadequate detection limits, which impairs identification. In this study, we achieved light element identification with nanoscale spatial resolution in a multi-component metal alloy through unsupervised machine learning algorithms of singular value decomposition (SVD) and independent component analysis (ICA). Improvement of the signal-to-noise ratio (SNR) in the STEM-EDS spectrum images was achieved by combining SVD and ICA, leading to the identification of a nanoscale N-depleted region that was not observed in as-measured STEM-EDS. Additionally, the formation of the nanoscale N-depleted region was validated using STEM–electron energy loss spectroscopy and multicomponent diffusional transformation simulation. The enhancement of SNR in STEM-EDS spectrum images by machine learning algorithms can provide an efficient, economical chemical analysis method to identify light elements at the nanoscale.

Transition from poor ductility to room-temperature superplasticity in a nanostructured aluminum alloy

Recent developments of nanostructured materials with grain sizes in the nanometer to submicrometer range have provided ground for numerous functional properties and new applications. However, in terms of mechanical properties, bulk nanostructured materials typically show poor ductility despite their high strength, which limits their use for structural applications. The present article shows that the poor ductility of nanostructured alloys can be changed to room-temperature superplastisity by a transition in the deformation mechanism from dislocation activity to grain-boundary sliding. We report the first observation of room-temperature superplasticity (over 400% tensile elongations) in a nanostructured Al alloy by enhanced grain-boundary sliding. The room-temperature grain-boundary sliding and superplasticity was realized by engineering the Zn segregation along the Al/Al boundaries through severe plastic deformation. This work introduces a new boundary-based strategy to improve the mechanical properties of nanostructured materials for structural applications, where high deformability is a requirement.

Concentration depth distribution of grain boundary segregation measured by wavelength dispersive X-ray spectroscopy

A method using wavelength dispersive X-ray spectroscopy (WDS) is applied to the measurement of grain boundary segregation of Sn in silicon steel. The quantification of monolayer concentration of Sn is acquired, which demonstrates an obvious segregation of Sn at grain boundaries. In consideration of the fact that segregated impurities (Sn or other species) distribute in multilayer and not just monolayer segregation can be characterized by WDS, the Gaussian distribution is applied to formulate the multilayer concentration depth distribution according to the measured total concentration. A correction factor is then put forward to improve the quantification. Based on the measured segregation of Sn and the derived formula of multilayer concentration depth distribution, the grain boundary concentrations of Sn are calculated for different thicknesses of segregated layer. From the experimental measurement, theoretical analyses and calculated results, an effective approach for the research of grain boundary segregation is provided.

Segregation of Yttrium at Grain Boundaries in α-Al2O3

Quantitative X-ray analysis allows the investigation of yttrium-doped grain boundaries. In the present study well-defined bicrystal interfaces were characterized. The quantitative comparison of segregation at different bicrystals requires a correction of artifacts in the X-ray spectra due to mass absorption, fluorescence, and beam spread. Mean grain boundary excess values of 3 Y/nm2 and around 5 Y/nm2 were found at a ∑17 and ∑37 symmetrical grain boundary, respectively. Additionally, with the ∑17 bicrystal YAG precipitation and presence of silicon was found.

Linear least-squares fit evaluation of series of analytical spectra from planar defects: extension and possible implementations in scanning transmission electron microscopy

In a previous paper, a new technique was introduced to determine the chemistry of crystallographically well-defined planar defects (such as straight interfaces, grain boundaries, twins, inversion or antiphase domain boundaries) in the presence of homogeneous solute segregation or selective doping. The technique is based on a linear least-squares fit using series of analytical (electron energy-loss or energy-dispersive X-ray) spectra acquired in a transmission electron microscope that is operated in nano-probe mode with the planar defect centred edge-on. First, additional notes on the use of proper k-factors and determination of Gibbsian excess segregation are given in this note. Using simulated data sets, it is shown that the linear least-squares fit improves both the accuracy and the robustness to noise beyond that obtainable by independently repeated measurements. It is then shown how the method originally developed for a stationary nano-probe mode in transmission electron microscopy can be extended to a focused electron beam that scans a square region in scanning transmission electron microscopy. The necessary modifications to scan geometry and corresponding numerical evaluation are described, and three different practical implementations are proposed.

The quantitative analysis of thin specimens: a review of progress from the Cliff-Lorimer to the new zeta-factor methods

A new quantitative thin-film X-ray analysis procedure termed the zeta-factor method is proposed. This new zeta-factor method overcomes the two major limitations of the conventional Cliff-Lorimer method for quantification: (1) use of pure-element rather than multielement, thin-specimen standards and (2) built-in X-ray absorption correction with simultaneous thickness determination. Combined with a universal, standard, thin specimen, a series of zeta-factors covering a significant fraction of the periodic table can be estimated. This zeta-factor estimation can also provide information about both the detector efficiency and the microscope-detector interface system. Light-element analysis can also be performed more easily because of the built-in absorption correction. Additionally, the new zeta-factor method has several advantages over the Cliff-Lorimer ratio method because information on the specimen thickness at the individual analysis points is produced simultaneously with compositions, thus permitting concurrent determination of the spatial resolution and the analytical sensitivity. In this work, details of the zeta-factor method and how it improves on the Cliff-Lorimer approach are demonstrated, along with several applications.

Identification of intergranular phases in ceramic nanocomposites

Analytical FEG-TEM was used for nanostructural and nanochemical characterization of Al2O3-TiN (composite I) and Si3N4-TiN (composite II) ceramic composite systems. The presence of vitreous intergranular phases in pockets at multiple grain junctions and in thin films ( approximately 0.8 nm thick) at grain boundaries was revealed by high resolution and Fresnel fringe imaging techniques. The existence of a Ti-rich thin intergranular film at alumina grain boundaries was revealed by EDS line-scanning across internal interfaces at the 1.5 nm lateral resolution level. Extracting interface specific information at subnanometre levels by means of quantitative spatial difference EELS allowed an identification of intergranular phases. Ti sub-oxide existed in thin films at Al2O3 and TiN grain boundaries, whereas a mixed Al-Ti-O-N glassy phase was observed in pockets at triple grain junctions in composite I. In composite II, residual siliceous oxide and oxynitride glass phases were identified in thin films at Si3N4 grain boundaries and multiple grain junctions, respectively. These observations indicated that the chemistry of the intergranular phase in thin grain boundary films is notably different from that in larger pockets at multiple grain junctions.